1. Technical Field
The present disclosure relates generally to radio frequency (RF) signal circuitry, and more particularly, to spur cancellation in GSM-GPRS-EDGE power amplifiers with DC-DC converters.
2. Related Art
Wireless communications systems find application in numerous contexts involving data transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Chief amongst these systems with respect to popularity and deployment is the mobile or cellular phone, and it has been estimated that there are over 4.6 billion subscriptions worldwide.
Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System) that includes third generation (3G) modalities such as W-CDMA (Wideband Code Division Multiplexing), and fourth generation (4G) such as LTE (Long Term Evolution).
A fundamental component of mobile handsets, or any wireless communications system for that matter, is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modulates it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals. Conventional mobile handset transceivers typically do not generate sufficient power or have sufficient sensitivity for reliable communications standing alone. Thus, additional conditioning of the RF signal is necessary. The circuitry between the transceiver and the antenna that provide this functionality is referred to as the front-end module, which include a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity.
Ultimately, all of the components of a mobile handset are powered from a battery with a peak output of approximately 4 volts, but different components have varying voltage supply requirements. Moreover, the common rechargeable lithium-ion batteries slightly decrease in output voltage as energy is depleted. Thus, conventional power amplifiers of CDMA front end circuits utilize DC-DC converters to increase and maintain the supply voltage at the required levels. The need for higher efficiency power conversion has been more pronounced in CDMA/WCDMA applications rather than GSM/GPRS/EDGE applications because the former has a greater power output dynamic range. Additionally, CDMA/WCDMA handsets consume more power because the on-board central processors execute more instructions and operate at higher data rates. On the other hand, first and second generation GSM handsets, which utilize 0.25-0.35 μm Silicon integrated circuit substrate technology, could not achieve significant power consumption reductions at the power amplifier level because the digital and analog baseband circuitry, as well as RF modulation circuitry, were more significant power drains in comparison. Additionally, single-slot GSM/GPRS/EDGE operation restrictions imposed by earlier mobile network infrastructure contributed to reduced power consumption by the headset devices.
With GSM/GPRS/EDGE handsets, there are primarily two different modalities of controlling the output power of the RF power amplifier with a fixed RF input power. One modality is base/gate current or voltage control, and the second modality is collector/drain voltage control, depending on the transistor technology utilized, e.g., bipolar junction or heterojunction bipolar, or metal-semiconductor field effect, metal oxide semiconductor field effect, or high electron mobility. In either configuration, power is applied to the base of each RF transistor stage through a resistor or a current mirror circuit. With most implementations, the transistors of the power control circuitry at low to mid RF output power levels consumes battery power, decreasing efficiency. To the extent any earlier devices addressed power amplifier power source efficiency, those attempts were focused on maximum output power levels.
However, in most urban environments, the average distance to a base station is not so extensive as to constantly require high power outputs. Indeed, according to the GSM Gaussian minimum shift keying (GMSK) probability distribution function, the output power at the handset antenna is 18 dBm to 27 dBm a majority of the time. Thus, talk times can be significantly improved with greater power supply conversion efficiencies at low to mid-level power outputs. Accordingly, there is a need in the art for input voltage controlled GSM/GPRS/EDGE power amplifiers having greater efficiency across all output levels.
There are several deficiencies associated with utilizing the aforementioned DC-DC converters in GSM/GPRS/EDGE operating modes that heretofore was insurmountable. As is well understood, the output of DC-DC converters has voltage ripples that vary depending upon inductance and capacitance values of the components chosen for the output filter needed to meet voltage settling time requirements. In general, the lower the output voltage level, the higher the relative voltage ripple. Additionally, the voltage ripple is dependent on the conversion mode of the DC-DC converter, e.g., pulse-width modulation (PWM), pulse-frequency modulation (PFM), skip mode, and so forth. The power control curve of conventional GSM power amplifiers, whether controlled through the base/gate or through the collector/drain, has variable control voltage sensitivities. That is, at lower RF output levels, a lower voltage increase is needed for the same increase in output power as is needed for higher RF output power levels. The foregoing performance characteristics of DC-DC converters result in an increase of local oscillation therein, which produces increased level of spurs in the mid to low power output levels of the power amplifier.
The voltage ripples at the output of the DC-DC converts, in turn, results in an amplitude modulation being applied to the RF signal. Thus, spur spectrum components appear at the local oscillator frequency offsets from the main RF signal. These include the continuous wave signal spur, as well as others across a wide spectrum that depends on the DC-DC converter operation mode and the selected output filter components. These spur signals may be at such levels that violate the strict spectrum purity requirements as defined by the GSM standard (e.g., GSM05.05).
There is accordingly a need in the art for adjustable DC-DC converters for GSM/GPRS/EDGE power amplifier power control applications, but which also satisfy maximum operating current and high speed voltage ramping requirements, while minimizing noise. More particularly, there is a need for DC-DC converters that utilize a higher local oscillator frequency than the RF signal bandwidth, (which for GSM/GPRS/EDGE is 200 kHz) while still retaining values and sizes of external filter components that are suitable for use in handset applications.